Question

Whether the diffraction effects from a slit will be more clearly visible or less clearly, if the slit-width is increased?

Answer 1

As a result there is considerable overlap of the diffraction patterns and hence wide fringes are observed. But, if the width of slits are increased, most light pass through the slits undiffrcted. Hence there is less diffraction. As a result the overlap due to diffraction is less and the fringes become narrow and lesser in number.
In case of interference, fringe width doesn't depend upon width of the slits, but only depends upon distance between the two slits. So, increasing width of the slits, does not make a difference in width of fringes or their visibility. But, if we will make the slits wider than the wavelength of the light used, then we will not see any interference pattern at all.